Carbon nanotubes (CNTs) are arousing great interest in the field of research, both fundamental and applied, as their properties are exceptional in many respects. From a mechanical view point, CNTs exhibit both an excellent stiffness, comparable to that of steel, while being extremely light (6 times lighter than steel). CNTs also exhibit a good thermal and electrical conductivity. CNTs have already been proposed as reinforcements in composite materials.
Despite the highly advantageous properties of CNTs, to date, their use in reinforcing the structures of composite materials has proved to be not very satisfactory. This is because little or no improvement in the mechanical properties of the composite material, such as, for example, the tensile, flexural and compressive strength, stiffness and lifetime, lightening of the density, corrosion resistance, was obtained. Furthermore, the improvement in the electrical and/or thermal conduction properties was insufficient. This can be explained, for example, by the deterioration of the CNTs or of their properties during the dispersion of the CNTs, by the poor dispersion or alignment in the matrix of the composite material, by the high contact resistance between the CNTs and/or between the CNTs and their environment (matrix, substrates, and the like), by the addition of surfactants/dispersants, by an insufficient interface between the CNTs and the matrix, or by the use of a high content of CNTs.
One alternative consists in using conventional reinforcements, such as, for example, particles and fibers of silicon carbide (SiC), of alumina (Al2O3), of carbon fibers, and the like, at the surface of which carbon nanotubes (CNTs) are synthesized.
The document WO 2010/066990 describes in particular the synthesis of CNTs at the surface of reinforcements by using a carbon source consisting of xylene and acetylene and a ferrocene-comprising catalyst.
However, the growth, the density and the morphology of the CNTs are not very satisfactory and the weight yields are close to 10%.
There thus exists a real need for a process for the synthesis of CNTs at the surface of a material, in particular a material which can be used as reinforcement, for example in composite materials, which overcomes the failings, disadvantages and obstacles for the prior art, which can be carried out industrially and which is economically advantageous.
Furthermore, there exists a real need to have available a process for the synthesis of CNTs at the surface of a material, in particular a material which can be used as reinforcement, for example in composite materials:                which can be suitable for the various types and geometries of materials/reinforcements to be treated (short fibers, long fibers, particles, and the like);        which makes possible a homogeneity, in particular in diameter, in density and in arrangement of the deposited CNTs;        which ensures good yields for the synthesis of the CNTs;        which makes it possible to adjust the parameters of the process in order to adapt the homogeneity, the diameter and the density of the CNTs to the targeted application.        the CVD synthesis temperature adapted to the nature of the materials/reinforcements to be treated.        
It has thus been found, entirely surprisingly, that the fact of mixing first micrometric reinforcements in accordance with the invention and subsequently of bringing about the growth of CNTs by aerosol CVD on these same mixed reinforcements, in particular Al2O3 (alumina) and SiC (silicon carbide) microparticles, makes it possible to greatly improve the yield for the synthesis of the CNTs.